High-performance, transparent cured products comprising curable epoxy compositions are used in a wide variety of fields, including electric and electronic applications, optical applications, aerospace and automotive applications, and various other fields. More specific uses for the curable epoxy compositions include use as sealing materials, laminating adhesives or plates, coating compositions, encapsulants and/or adhesives for electric and electronic materials, fiber optic and optical semiconductor components, sensors and various other applications.
By way of example only, transparent epoxy resin compositions have uses in optical microelectronics, such as sensors and light emitters. These include the families of light-emitting diodes (LEDs), light and image sensors, and opto-couplers used to isolate microelectronic circuits from voltage spikes. These optoelectronic devices have been miniaturized and optimized for mass production through complex supply chains for the manufacture of many end-use applications. The industry has adopted the Joint Electronic Device Engineering Council (JEDEC) Solid State Technology Association's standardization pertaining to Moisture Sensitivity Levels (MSL) to ensure that electronic components are suitable for use in sophisticated manufacturing networks, typically requiring a minimum MSL level 3 or 4. Many, if not most, semiconductor devices require the highest level of robustness, i.e., MSL 1.
MSL is an indication whether a device may be stored in a hot and humid environment, assembled into a circuit board and then flash heated at 260 degrees)(° C. for soldering without causing the device to crack or apply damaging stress to its encapsulated electronic elements. Neat epoxy resins perform poorly with respect to MSL because epoxy will absorb moisture and they have a coefficient of thermal expansion (CTE) that is typically much higher than those of the encapsulated electronic devices. By way of example, the CTE for a typical epoxy is 80 parts per million/degree Celsius (ppm/° C.), whereas the CTE for copper is about 16.7 ppm/° C. and that for silicon is about 2.6 ppm/° C.
It is well known in the art to modify epoxy resins intended for electrical encapsulation with inorganic fillers (such as silica and metal oxides) in order to reduce the CTE and moisture absorption (MSL) of the resulting composition. Manufacturers of clear epoxy for optoelectronic devices have also utilized clear inorganic fillers but heretofore, these have yielded cured encapsulants with greatly diminished regular transmission of light (RTRAN). Thus far, encapsulation materials utilizing clear fillers may appear translucent and yield good total transmission of light (TTRAN), but the filler causes light scattering that dramatically reduces straight line transmission (RTRAN). This could be due to poor coupling between the epoxy matrix and the filler, poor clarity of the filler itself, or because the filler and the epoxy have a mismatched index of refraction.
Many optoelectronic devices require a high level of RTRAN, or regular light transmittance, to be effective (for example, often >80%). However, due to the poor optical properties of traditional epoxy plus filler compositions, the industry has been limited to the use of neat epoxy compounds or epoxy substitutes that introduce other limitations upon design and manufacturing possibilities. Accordingly, there is a chronic and growing need for an epoxy resin encapsulant that produces high RTRAN, robust MSL (e.g. <4) and low CTE. Furthermore, although in alternate applications (i.e., for example automotive sensors) a minimum regular light transmittance (RTRAN) of 30-40% may be sufficient, these applications still require both a robust MSL (e.g. <4) and low CTE.
The present disclosure provides a curable encapsulant, namely a polymer composite encapsulant or composition, which is ideally suited for a variety of applications which require a high-performance encapsulant that can withstand high temperature cycling and, thus, avoids imparting high thermally-induced stresses on the components it is encapsulating, laminating, adhering, etc. In other words, an encapsulant composition that provides for a cured product with a low CTE and robust MSL. In certain embodiments, the cured encapsulant, once cured in its “clear” variant, has a coefficient of thermal expansion (CTE) of less than 50 parts per million/degree Celsius (ppm/° C.), when measured at temperatures between 100°−120° C., and a regular light transmission (RTRAN) greater than about 30% over light wavelengths from about 350 nm to about 2500 nm in specimens 0.5 and 1.0 mm thick. In alternate, preferred embodiments, the cured encapsulant product has a CTE equal to or less than 30 ppm/° C., when measured at temperatures between 100°−120° C., and provides RTRAN greater than 65% over light wavelengths from 350 nm into the infrared (i.e. 2500 nm) when measured in molded specimens 0.5 and 1.0 mm thick.
In one embodiment of the invention, the curable encapsulant composition of epoxy-resin comprises triglycidyl isocyanurate (TGIC), a curing agent, a ring opening agent, and inorganic filler materials, wherein the inorganic filler material is selected from the group consisting of silica glass compositions (or mixtures thereof) with an index of refraction between 1.50 and 1.52 (e.g., about 1.515). More specifically, in embodiments of the invention, the silica glass composition comprises at least one soda lime glass composition or mixtures thereof.
It is also desirable, in some embodiments, to add modifiers, such as an accelerator, an antioxidant, silane, UV stabilizers, and/or a mold release agent to the epoxy-resin composition so that the composition is well adapted for use in transfer molding, injection molding, or compression molding processes used in the microelectronics and semiconductor industry.